1. Field of the Invention
The invention relates to a process for the manufacture of a shaped bar. The invention also covers a device suitable for carrying out the process, as well as use of the process and use of the device.
2. Discussion of the Prior Art
One known process for the manufacture of metal profiles is extrusion. However, with current extrusion technology, it is very difficult to manufacture large Aluminium alloy profiles with a width of more than approximately 700 mm. Another disadvantage consists in that it is very difficult to obtain profile wall thicknesses of less than approximately 2 mm. However, in view of weight and cost savings, it would be highly desirable to reduce the wall thicknesses of profiles, i.e. to achieve all thicknesses of less than 1 mm while still observing the usual geometric profile tolerances.
The limited extrusion force and the limited possibilities of obtaining uniform metal distribution with respect to, temperature and flow rate are the essential factors preventing the manufacture of extremely thin-walled profiles using current extrusion technology.
However, in current extrusion technology, certain limits exist even in the manufacture of profiles of medium or small width, with respect to the materials than can be processed and the cross-sectional dimensions to be produced. For example, it is virtually impossible or very difficult to press hard Aluminium alloys with the extrusion forces normally used in conventional extruders. This limitation applies in particular to the manufacture of hollow profiles, particularly multi-compartment hollow profiles. The resulting slow extrusion rate has a negative effect on production costs. In addition, the dimensional tolerances are often insufficient and there is often poor mental distribution, noticeable above all through insufficient mould filling in shaped parts with small metal crossectional dimensions.
The extrusion of particle-reinforced composite materials consisting of a metal matrix with particles of fibres of non-metallic, high-melting materials dispersed therein leads to comparable problems to the above-mentioned processing of hard alloys. The manufacture of these so-called Metal Matrix Composites is described in detail in WOA-87/06624, WOA-91/02098 and WOA-92/01821. The particles to be introduced into the metal matrix are first essentially introduced homogeneously in an alloy melt and the molten composite material is then case, e.g. by continuous casting, into the format suitable for further processing by extrusion of rolling.
A process of the type mentioned at the outset is known from JP-A-04066219. The aim of the invention is therefore to provide a process of the type mentioned at the outset and a device suitable for carrying out the process, by means of which hard alloys and composite materials of all types can be processed into high-quality products in a cost-effective manner. Another aim is the economical manufacture of extremely thinwalled large profiles and/or large profiles of extreme width. In addition, it should be possible to modify existing extrusion installations in a simple and cost-effective manner.
Pursuant to the present invention, preform is usually inserted in the form of billet into a preform chamber which will be described in more detail hereinbelow. The preform and the preform chamber therefore correspond to the extrusion billet and the container in extrusion.
By virtue of the fact that the preform is shaped in the partially solid/partially liquid state according to the invention, materials which were virtually impossible to manufacture of could only be manufactured in a very uneconomical manner by conventional extrusion can be processed into profiles with a constant extrusion force. As a result of the low extrusion forces required, comparable profile dimensions can be pressed in smaller installations than in the case of conventional manufacturing methods, this being advantageous from the point of view of production costs.
One essential advantage of the process according to the invention is that hard alloys and composite materials can be processed into profiles with metallurgical properties that cannot be obtained by conventional extrusion.
Wider profiles with smaller profile wall thicknesses than in possible with current extrusion technology can also be manufactured by the process according to the invention.
The central idea underlying the process according to the invention consists in bringing the preform so close to the final cross section with the lowest possible extrusion force that the final shaping of the cross section of the shaped bar can also be carried out with low extrusion force by means of a die. This is achieved by the shaping in the partially solid/partially liquid state according to the invention.
Compared to the use of conventional perfectly set extrusion billets, the use of preforms in the partially solid/partially liquid state has the advantage that shaping can be carried out with substantially lower extrusion force. If the liquid phase fraction is kept low compared to the solid phase fraction, sufficiently rapid setting can also be achieved in thick-walled profile regions.
As the pressure applied to the preform, i.e. the extrusion force, cannot be increased as desired, e.g. as a result of the high container temperature of up to 600xc2x0 C. required in the case of special additives, in an advantageous development of the process according to the invention, the preform is pressed to form the shaped bar with the aid of a tensile force acting on the shaped bar.
The degree of shaping upon the transition of the preform to the shaped bar in the partially solid/partially liquid state is preferably at least 50%, preferably at least 80%. The degree of shaping refers here to the reduction in the cross section during the shaping of the preform to form the shaped bar.
If the shaped bar has to have a high surface quality and/or high dimensional tolerance, the shaped bar can be guided through a die immediately after it emerges from the mould for the final shaping of the cross section of the shaped bar. This final shaping of the cross section of the shaped bar is advantageously carried out with shaping of no more than 15%, preferably no more than 10%.
After it emerges from the mould or the die, the shaped bar is preferably cooled by the complete evaporation of a coolant sprayed on to the shaped bar. Cooling with complete evaporation of the coolant prevents liquid coolant from being able to flow back in the direction of the hot metal possibly still in the partially liquid state. By virtue of this measure, the cooling means can be arranged as close as possible to the site of the desired cooling, i.e. as close as possible to the mould or the die.
The liquid phase fraction in the preform during the shaping thereof depends on the nature of the material to be processed. In general, this fraction is no more than 70%, and is preferably approximately 20 to 50%. In principle, any materials in which a partially solid/partially liquid state can be set within a sufficiently broad temperature interval for practical purposes can be used for the preforms. Examples of suitable materials are:
alloys, in particular aluminium and magnesium alloys in the thixotropic state, with different solid/liquid fractions, e.g. hard alloys of the AlMg or MgAl type,
alloys based on magnesium or copper in the thixotropic state, with different solid/liquid fractions, and
alloys based on aluminium or magnesium with metallic or non-metallic fractions of high-melting particles and/or fibres (Metal Matrix Composites).
Aluminium and magnesium alloys in particular are suitable as the metal matrix. Its basic properties, such as mechanical strength and elongation can be achieved in a known manner by means of the various types of alloy. The non-metallic additives can have an advantageous effect, inter alia, on hardness, rigidity and other properties. Preferred non-metallic additives are ceramic materials such as metal oxides, metal nitrides and metal carbides. Examples of materials of this kind are silicon carbide, aluminium oxide, boron carbide, silicon nitride and boron nitride.
In principle, profiles can be manufactured from composite materials in such a manner that the preform already contains all of the materials in the desired form. However, with the process according to the invention, a filler material can also be added to the preform in the partially solid/partially liquid state before it enters the mould. This filler material can be added indifferent forms and in different states of aggregation. E.g. the filler material can be supplied continuously to the preform in solid form as wire, fibres or powder. Wires, e.g. in the form of reinforcements can remain in the profile. However, a material which melts in the partially liquid/partially solid range, where it then alloys or triggers a chemical reaction can also be added in the form of wire. The filler material can also be added in the liquid state or in the gaseous state.
One essential advantage of the process according to the invention over conventional extrusion also consists in that preforms can be composed of cross-sectionally different material regions. E.g., the edge zone or even internal parts of a profile can be provided with different mechanical properties from those of the matrix, such as higher hardness, rigidity, abrasion resistance and the like.
Preforms with cross-sectionally different material regions can be processed in that the preform is guided through a heating zone before it is shaped to form the shaped bar and is set to a uniform solid/liquid ratio over the entire cross section of the shaped bar in the heating zone. To this end, a cross-sectionally different temperature profile can be set in the heating zone as a function of cross-sectionally different material regions.
A device suitable for carrying out the process according to the invention includes an optionally heatable preform chamber for receiving the preform, an optionally heatable forming chamber connected to the preform chamber for shaping the preform to form the shaped bar, and a chilled mould connected to the forming chamber for the setting of the shaped bar, wherein a die can optionally also be arranged immediately downstream of the mould for the final shaping of the cross section of the shaped bar.
An extractor means can be arranged downstream of the downstream of the device according to the invention in order to apply a tensile force to the shaped bar and therefore to assist the entire extrusion process. The extractor means can include grippers and/or drive rollers.
The wall of the forming chamber preferably passes over into the wall of the mould with a constant curvature, i.e. the cross section of the preform being shaped to form the shaped bar decreases continuously.
Heating lines are arranged in the preform chamber and/or in the forming chamber in order to produce or maintain the partially solid/partially liquid state of the preform. In addition, an intermediate layer of a heat-insulating material is advantageously arranged between the generally heated forming chamber and the chilled mould.
A heating means is advantageously arranged between the preform chamber and the forming chamber. This heating means preferably has individually heatable flow channels for the preform.
In a preferred embodiment of the device according to the invention, the heating means consists of at least two disc-shaped heating elements arranged side by side and provided with integrated heating conductors, the heating elements being individually controllable.
A direct cooling means is provided for further cooling of the shaped bar emerging from the mould or the die. For the aforementioned reasons, a cooling means with complete evaporation of the coolant applied to the shaped bar is preferred.
A particularly preferred application of the process and device according to the invention consists of the manufacture of profiles with cross-sectionally different material regions.